Project Summary More than 1.5 million infants each year are born with in utero exposure to HIV infection. While prevention of mother to child transmission successfully prevents congenital infection, HIV exposed but uninfected infants (HEU) are more susceptible to common enteric and respiratory infections than their unexposed counterparts, and have higher mortality rate. The precise immunologic alterations that are responsible for this phenomenon among HEU infants have never been clearly characterized. Studies conducted after widespread availability of antiretroviral therapy (ART) have shown less pronounced differences in morbidity and mortality between HIV exposed and unexposed infants. We believe that the immune perturbations associated with in utero HIV exposure are mitigated by effective ART and the longer the control of the HIV infection is during gestation, the less immune alterations and clinical risk of disease the infant will experience. In this study, we will test the hypothesis that infants born to mothers with suppressed HIV infection since conception will have adaptive immune responses similar to HIV-unexposed infants while infants exposed to high level of HIV infection through most of pregnancy will have a dysregulated adaptive immune response. We propose a longitudinal analysis of adaptive immune subsets in HEU infants, comparing three well-characterized mother-infant cohorts from a single health center in Malawi, where, as in much of sub-Saharan Africa, women often receive their first HIV diagnosis when they present for antenatal care late in pregnancy. In this setting, we will enroll (1) infants born to women diagnosed with HIV infection at the first antenatal visit after 26 weeks gestation, thus exposed to uncontrolled viremia for over half of the pregnancy, (2) infants born to women on ART with undetectable viral loads before conception, and (3) HIV unexposed infants born to HIV uninfected mothers. All HIV-infected women will receive the same ART regimen and will breastfeed their infants during the 9-month follow up period. We will assess the adaptive immune response by probing the immune system at birth, 4 and 9 months of age with both polyclonal stimuli and routine immunization antigens, to which all infants will be exposed in the first three months of life. We will compare differentiation, activation levels and antigen specific T and B cell responses for the three groups of infants, applying state-of-the-art technology for detailed and comprehensive analyses. This will be the most comprehensive longitudinal assessment of the impact of HIV exposure on the development of the adaptive immune responses in infants. The results will provide important evidence for public health policy in helping to determine the optimal timing of ART treatment to maximize infant health and survival.